Wiktionary says it is either likely a pseudo-condensed pronunciation of kilometer or onomatopoeic of the sound of a military odometer.

Though kilometers are not commonly used to measure distance in the USA, “klick” is commonly used by the US military, which uses the metric system almost exclusively in order to facilitate communication with allied forces.

I think the "communication" part refers to the use of the metric system, i.e. expressing themselves in kilometers instead of miles. Not the way they pronounce them. I don't have an answer to your question though.
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Mr ListerOct 7 '12 at 12:31

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The earliest use I find in Google Books is 1969, in this account of Vietnam operations. Might it have been a convenient, unambiguous abbreviation for radio communications?
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StoneyBOct 7 '12 at 12:44

@MrLister Oh I see thanks, perhaps the wording there was a bit confusing. Anyway I have removed that part of the question.
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z7sg ѪOct 7 '12 at 12:50

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@tchrist Heinlein was Navy, discharged in '34; the reference linked in my other comment takes the earliest citation back to Heinlein's 1959 Starship Troopers. This suggests an origin in the European theatre.
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StoneyBOct 7 '12 at 19:02

There's a story it was named by the Americans from the Australians' practice of using gas regulator knobs to keep track of distance, resulting in considerable clicking when reset at each km. There's another of kilometer seeming unwieldy to the Americans when talking with allied forces and locals in Berlin. Stories seen on the web somewhere don't make for a supported answer, though. Only the Australian explanation matches the dating of it to the war in Viet Nam.
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Jon HannaFeb 20 '13 at 21:27

5 Answers
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I was in the military, but not Viet-Nam era. From my own judgment, I don't think it has anything to do with odometers. I think it's a dark humor reference.

When sighting in an M-16, the sights are adjusted in clicks. Looking downrange at a calibrated 100 yard target, one click moves the projectile/impact one inch across the face of the target. (left/right/up/down)

In the same way, when looking at a topo map, one "klick" is one full increment line on the map.

So viewed from the sky, (the point of view of either God or an enemy bomber, however you wish to imagine it) when we go one kilometer, we've traveled one click across the face of the map. The unspoken implication is that in the end we all might just be points on someone else's target...

Simply my opinion, but that's how it struck me the first time I heard it.

The term arose from the use of forward observed non-line of sight artillery targeting and actually began with the United States Marine Corp during the interwar period between the Korean and Vietnam wars. The M19 mortar introduced at the time had dials for adjusting the azimuth and elevation. It produced a loud "click" sound when both dials where set to a particular combination that resulted in a point of impact 1000m away on level ground at standard temperature pressure. It was during this period that Marines began developing the co-ordination of light artillery with infantry manuever by a forward observer using field radio.

Just as background, the M19 was actually introduced to the US Army in 1942, arriving in the Marine Corps. arsenal sometime later, as a replacement for the M2 mortar, but proved inferior in several respects. It was openly derided by many Marines and several jokes about it's inaccuracy involved jokes about it's "click" sound.

I was once told by an old gunner that the spelling 'klick' came from a field manual written by a german ex-pat associated with development of the weapon system. There may be some measure of truth to this, but the claim is somewhat dubious.

This is a fascinating claim. Can you please support your answer with facts, references, or specific expertise? Thanks.
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MετάEdFeb 20 '13 at 21:09

There was a blurb about it on a display at the National Museum of the History of the Marine Corp at Washington Yard. It may be referenced at the new National Museum of the Marine Corps in Quantico, but I cannot substantiate that.
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OCDtechFeb 20 '13 at 21:53

MGRS, mentioned by quarkie, would have been developed about the same time if not in coordination. The NATO phonetic alphabet became standard for NATO forces in 1958. All three seem to corroborate each other since they all evolved immediately preceeding the Vietname era.
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OCDtechFeb 20 '13 at 22:11

It's slang and has use not just in military. It refers to older style odometers which produced a just audible click when a kilometer or mile distance passed in a car. Nowadays, with silent odometers, it's more common to refer to k's but, if you're talking to someone about their far holiday trip, you might say "So how many klicks did you do?"

It appears however that the use arose in the US military, during the Vietnam War.
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StoneyBOct 7 '12 at 16:16

@tchrist OP reports that onomatopeic origin in his first sentence.
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StoneyBOct 7 '12 at 18:20

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@tchrist There's a good discussion, with additional forward links, here. One discutant objects to this derivation because the odometers he is familiar with didn't in fact "click"; but I distinctly recall the click in my father's '54 Hillman. He also objects because US odometers aren't metric--to my mind a relevant but not insuperable point. My verdict would be "not proven".
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StoneyBOct 7 '12 at 18:55

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Every car we owned when I was a child, all of them 50-s or 60-s era, clicked the miles over audibly. (I still miss that...!) The first time I heard "klick" from my Army brother, in the late 70's, I automatically made the assumption that it was due to odometers, but he explained the same dark humor @Bob did.
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shiprOct 8 '12 at 18:07

Military grid reference system (MGRS) is metric, because it is used internationally within NATO. This may explain why US military use metric system for the distances on the ground. As for word itself, it is important for any military terms to be clearly distinguishable from the others on the radio. Kilometer can be easily confused with meter when radio is not very clear.

A click was used as a refernce on mortars, machine guns and some artillery early on in the military. The T&E mech. when moved 1 click would change the strike or impact 1 meter at a distance of 1000 meters. This info. found in the FM's on the above equipment.

One click or klick when talking about distance on a military map is 1000 meters or one KM